Gene Interactions – Explanation, Types & Significance

Gene Interactions 

Introduction

Gene is defined as the basic unit of heredity. It is made up of DNA and carries information useful for the formation of proteins and genetic variation. Every person has two copies of a gene.  

Genes decide the genotype and phenotype of an organism. Sometimes, the effects of one gene gets modified by the presence or absence of various other genes.  

In such a process, two or more types of genes can act simultaneously to modify the expression of the other gene. The expression of one gene depends on the presence or absence of another gene in any organism. 

Explanation

Mendel’s dihybrid cross: 

We know that Mendel simultaneously examined two different genes that controlled two different characters in his dihybrid cross with pea plant. 

For example, in one series of his experiments, Mendel began by crossing a plant that was homozygous for the traits round seed shape and yellow seed color (RRYY), with a different plant that was homozygous for the traits wrinkled seed shape and green seed color (rryy). 

When Mendel crossed the F1 progeny of the two plants with each (RrYr x RrYy), he obtained an F2 generation with a phenotypic ratio of  9: 3: 3: 1. 

In this dihybrid cross, each gene locus has an independent effect on a single phenotype. Thus, the R and r alleles affected only the shape of the seed and had no influence on the seed color, while the Y and y alleles only affected the seed color and had no influence on the seed shape. 

In this case, there are two separate genes that coded for two separate characteristics. But imagine what will happen if two different loci affect the same characteristics? 

What if, in Mendel’s experiment, both loci had an effect on seed color? 

When two genes are involved in the expression of one characteristic, a dihybrid cross can produce a phenotypic ratio that is very different from 9: 3: 3: 1. 

Under such conditions, there are more than two gene products that affect the same phenotype, and these products may have complex interactions. 

What are gene interactions? 

The interaction of genes present at different loci that affect the expression of the same character is known as gene interaction. 

It is the influence of alleles and non-alleles on the normal phenotypic expression of the genes. 

Types of Gene Interactions: 

Gene interactions are of two types: 

1. Inter-allelic or intra-genic gene interaction: 

In such cases, two alleles located on the same gene locus on two homologous chromosomes of a gene interact to produce phenotypic expression. 

Example: co-dominance, incomplete dominance, multiple alleles. 

Incomplete dominance: 

We have already studied that incomplete dominance is a phenomenon in which a dominant allele or a form of a gene does not entirely hide or mask the effects of a recessive allele, and the organism’s resulting phenotype shows a blending of both alleles.  

It is also known as semi-dominance or partial dominance.  

An example of incomplete dominance is feather color in chickens. A cross between a homozygous white chicken and a homozygous black chicken produces F1 chicken that are grey. If these grey F1 are intercrossed, they produce F2 birds in the ratio of 1 black: 2 grey: 1 white. 

Co-dominance: 

As opposed to incomplete dominance, co-dominance occurs when phenotypes of both parents are simultaneously expressed in the same offspring. 

In co-dominance, the expression of alleles is uniformly conspicuous, i.e., both the alleles have an equal chance of expressing themselves. 

An example of co-dominance is observed in the ABO blood groups of humans. The alleles A and B are expressed as A or B molecules present on the surface of red blood cells.  

The homozygotic forms (𝐼^𝐴 𝐼^𝐴 and 𝐼^𝐵 𝐼^𝐵 ) express either the A or the B phenotype, and heterozygotes (𝐼^𝐴 𝐼^𝐵) express both phenotypes equally. 

The 𝐼^𝐴 𝐼^𝐵 individual has blood type AB. In a self-cross between heterozygotes expressing a co-dominant trait, the three possible offspring genotypes are phenotypically distinct.  

In this case, the F2 genotype and phenotype ratio are the same, i.e., 1: 2: 1. Thus, the mendelian monohybrid ratio of  3: 1  gets modified. 

Multiple alleles: 

Individuals usually have only two alleles for each gene. Within a population, however, there may be three or more alleles for a single gene trait. Such a trait is termed multi-allelic.  

For example, in humans, the ABO blood group is a good example of a single gene that has multiple alleles. 

Blood cells have molecular markers on their surfaces, and these play an important role in allowing a person’s own body cells to be recognized by the immune system as ‘self ’. 

To represent multi-allelic blood groups using correct genetic notation, the gene is denoted as I and the three alleles are represented by superscripts: 𝐼^𝐴, 𝐼^𝐵   and 𝐼^𝑖.  

Alleles A and B are codominant, as they each produce a molecular marker on red blood cells. If both alleles are present, the blood cells have both markers. The i allele produces no molecular marker on the red blood cells and is recessive to both A and B.  

As a result, there are four possible phenotypes for the ABO blood system: a person may have blood group A, B, AB or O. There are, however, six possible genotypes. 

Another example of a gene that has multiple alleles is the gene for coat color in rabbits. There are four alleles, called normal, chinchilla, Himalayan and albino. 

In rabbits, there are four alleles which code for coat color: C, 𝑐^h, 𝑐^h and c. Allele C is dominant to all the other alleles and produces a full color coat.  

Allele c is recessive and this leads to an albino phenotype when the genotype is homozygous recessive. 

Allele 𝑐^ch  is dominant to 𝑐^h  allele, and 𝑐^h   is dominant to c allele. The dominance hierarchy can be written as C > 𝑐^ch > 𝑐^h > c. 

Full color coat is dominant over chinchilla, which is dominant over Himalayan, which is dominant over albino. 

Significance of multiple alleles: 

Multiple alleles increase the number of genotypes and phenotypes that can be produced. Without multiple-allele dominance, two alleles, such as T and t, create only three possible genotypes—in this example, TT, Tt, and tt—and two possible phenotypes.  

However, the four alleles for rabbit-coat color produce ten possible genotypes and four phenotypes. More variation in rabbit coat color comes from the interaction of the color gene with other genes, such as the agouti gene or the broken gene. 

The second type of gene interaction is: 

2. Non-allelic or inter-genic gene interaction: 

In such cases, two or more independent genes present on the same or different chromosomes interact to produce a new expression. 

Example: Epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc. 

Summary

• Gene is defined as the basic unit of heredity.
• Genes decide the genotype and phenotype of an organism. Sometimes, the effects of one gene gets modified by the presence or absence of various other genes.
• The interaction of genes present at different loci that affect the expression of the same character is known as gene interaction. It is the influence of alleles and non-alleles on the normal phenotypic expression of the genes.
• Gene interactions are of two types:

1. Inter-allelic or intra-genic gene interaction:

• In such cases, two alleles located on the same gene locus on two homologous
  chromosomes of a gene interact to produce phenotypic expression.
• Example: co-dominance, incomplete dominance, multiple alleles.
• Incomplete dominance is a phenomenon in which a dominant allele or a form of a gene does not entirely hide or mask the effects of a recessive allele, and the organism’s resulting phenotype shows a blending of both alleles.
• Co-dominance is a mode of inheritance in which both the alleles of a gene pair in a heterozygote are fully expressed. As a result, the phenotype of the offspring is a combination of the phenotype of the parents.
• Individuals usually have only two alleles for each gene. Within a population, however, there may be three or more alleles for a single gene trait. This is termed as multiple allelism.
• The second type of gene interactions is:

2. Non-allelic or inter-genic gene interaction:

• In such cases, two or more independent genes present on the same or different
  chromosomes interact to produce a new expression.
• Example: epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc